High-efficiency degradation of norfloxacin by Co–N co-doped biochar synergistically activated peroxymonosulfate: experiments and DFT calculations

Abstract

Co–N co-doped materials have been widely used in PMS systems for organic pollutant degradation. However, the synergy effect of Co and N in PMS activation processes is not fully understood. In the present study, a catalyst comprising Co nanoparticles incorporated within N-doped biochar (Co@NBC) was synthesized to facilitate the activation of PMS and the degradation of norfloxacin (NOR). The results demonstrated that the degradation efficiency of NOR in the Co@NBC-PMS system was 94.45% (0.341 min⁻¹). Quenching tests, EPR analyses, electrochemical measurements, and DFT calculations indicated that large amounts of graphitic N in Co@NBC remarkably augmented the direct electron transfer (DET) process. Furthermore, oxidized graphitic N stretched the O–H bond of PMS and promoted the generation of SO₅˙⁻, which was the main intermediate product for the formation of ¹O₂. In addition, graphitic N could facilitate the conversion of Co^II to Co^IVO by elongating the S–O bond of PMS and shortening the Co–O bond. Besides, reduced pyridinic N encouraged the reduction of Co^IVO to Co^III and Co^II. This valence state cycle of Co^II-Co^IVO-Co^III co-promoted by graphitic N and pyridinic N significantly enhanced the PMS activation process. Moreover, the possible degradation pathways of NOR and the environmental impact of its intermediates had been investigated through DFT calculations, LC-MS, and ECOSAR. Importantly, the developed system exhibited efficient removal of NOR under inorganic ion influence and in practical water matrices. This study not only developed an efficient PMS activation system (Co@NBC-PMS) for the remediation of antibiotic-contaminated wastewater, but also offered novel insights into the synergy effect of Co and N on activation mechanisms.